5. The current flows from the tungsten electrode to the work surface, and the
positively charged argon gas ions flow from the work surface to the tungsten
which puts too much heat into the metal and causes the base metal
underneath the oxide layer to liquefy while the surface remains hard and
impenetrable.
The electrode is connected to the negative terminal of the power supply.
Electrons are emitted from the tungsten electrode and accelerated while
traveling through the arc. A significant amount of energy, called the work
function, is required for an electron to be emitted from the electrode. When the
electron enters the workpiece, an amount of energy equivalent to the work
function is released. This is why in GTAW with DCEN more power (about two-
thirds) is located at the work end of the arc and less (about one-third) at the
electrode end. Consequently, a relatively narrow and deep weld is produced.
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6. Direct current electrode posititive (DCEP) solves the oxide problem because
the current flows from the workpiece to the tungsten, lifting the oxide off the
material in the arc zone. DCEP alone provides the oxide cleaning action and
very little penetration. Because the heat is concentrated on the tungsten
instead of the workpiece, DCEP also causes the tungsten to ball up at the end.
Direct-Current Electrode Positive (DCEP) This is also called the reverse
polarity. The electrode is connected to the positive terminal of the power
source. The heating effect of electrons is now at the tungsten electrode rather
than at the workpiece. Consequently, a shallow weld is produced.
Furthermore, a large-diameter, water-cooled electrodes must be used in order
to prevent the electrode tip from melting. The positive ions of the shielding gas
bombard the workpiece, knocking off oxide film and producing a clean weld
surface. Therefore, DCEP can be used for welding thin sheets of strong oxide-
forming materials such as aluminum and magnesium, where deep penetration
is not required.
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7. AC, then, combines DCEN and DCEP to provide good heat penetration with
cleaning action.
The oxide is sandblasted away – as you can see in the following pictures.
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11. Historically, though, AC has posed an obstacle to GTAW because the arc
frequently extinguishes itself as the current reaches a zero point before
reversing directions. Without any current passing between the tungsten and
the base metal, the arc simply goes out and affects the quality of the welding
arc – and the quality of the weld.
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12. The introduction of Squarewave technology brings a new level of TIG-welding
performance to the market.
Improvements in transformer-based GTAW machines created the square
wave, which increased the amount of time the arc spends at full-current flow
in both DCEN and DCEP. Square-wave technology eliminated the tendency
for the arc to extinguish when the current came to a halt as it reversed
directions by making the transition very quickly. This greatly improved the
stability of the arc and made square-wave technology the preferred method
for GTAW of aluminum and other materials that form an oxide layer, such as
magnesium.
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13. This wave is balanced with both cleaning and penetration.
Independent amperage (or amplitude) control allows the EP and EN
amperages to be set independently. This precisely controls heat input into the
work and even takes heat off the electrode. The EN portion of the cycle
controls the level of penetration, and the EP portion affects the arc cleaning
action.
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14. Here, you can see the difference in welds between maximum cleaning and
maximum penetration.
A current with greater EN than EP creates a narrow bead with deeper
penetration and no visible cleaning action, ideal for fillet welds and automated
applications. A current with greater EP than EN gives the operator a wider
bead with less penetration and clearly visible cleaning action, ideal for buildup
work
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15. This controls the amount of arc-cleaning action and width of the arc-etching
zone around the weld.
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16. The second major revolution in frequency technology came with the invention of the
inverter, which created the ability to increase or decrease output frequency beyond
the standard 60 Hz, which is the standard frequency delivered to every outlet in the
U.S. (other countries, such as Germany, England, and France, deliver AC power at
50 Hz). The inverter also allowed for the development of the advanced square wave,
which decreases the time it takes for the current to reverse directions, increasing arc
stability even more and eliminating the need for continuous high frequency.
Increasing frequency above 60 Hz causes the current to change direction more often,
which means that it spends less time per cycle in both DCEN and DCEP mode. By
spending less time at each polarity, the arc cone has less time to expand.
An arc cone at 400 Hz is much tighter and more focused at the exact spot the
electrode is pointing than an arc cone operating at 60 Hz (see Figure 1). The result is
significantly improved arc stability, ideal for fillet welds and other fit-ups requiring
precise penetration.
Combined with adjustable balance control to increase the electrode negative polarit
y—resulting in deeper penetration and tungsten that doesn't ball up—high AC
frequency can weld very tight joints with good penetration and without the risk of
laying down too much filler metal. Workpieces with wide gaps to fill or that require
buildup will benefit from the softer, wider arc cone that results from lower frequencies
Unlike other types of waveform control, such as balance and amplitude, frequency
control provides good penetration at both low and high frequencies. The primary
difference between the two is the width of the arc cone and resulting weld bead.
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17. Controls the width of the arc cone. Increasing the AC Frequency provides a
more focused arc with increased directional control.
Decreasing the AC Frequency softens the arc and broadens the weld puddle
for a wider weld bead.
A Good starting point for working with adjustable frequency for the first time is
between 80 and 120 Hz.
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18. At 60 Hz, you can see the bead doesn’t quite penetrate the thick aluminum.
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19. At 200 Hz, the bead is much tighter and penetrated the thicker metal.
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22. Arc shaping capabilities are enhanced by improved balance control. On the
left is the tungsten with a balled end, due to more time spent in the electrode
positive (EP) part of the cycle, which creates shallower penetration. On the
right is the tungsten with a sharp end, due to more time spent in the electrode
negative (EN) part of the cycle, which creates deeper penetration and allows
faster travel speeds.
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24. The four different waveforms affect the arc and puddle characteristics and the
penetration profile.
An advanced square wave (A) allows for fast travel speed. The advanced
square wave
waveform offers fast transitions between EN and EP for a
responsive, dynamic, and focused arc with better directional control. It forms a
fast-freezing puddle with deep penetration and fast travel speeds.
A soft square wave (B) provides maximum puddle control. Soft square wave
provides a smooth, soft, "buttery" arc with a fluid puddle and good wetting
action. The puddle is more fluid than with advanced square wave and more
controllable than with sine wave.
The sine wave (C) permits welding with traditional characteristics. The sine
wave offers a soft arc with the feel of a conventional power source. It provides
good wetting action and actually sounds quieter than other waves. Its fast
transition through the zero amperage point also eliminates the need for
continuous high frequency.
The triangular wave (D) reduces heat input. The triangular wave offers peak
amperage while reducing overall heat input into the weld. This leads to quick
puddle formation, low weld distortion, and fast travel speeds. It is especially
good for welding thin aluminum.
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27. Independent AC Amperage Control allows the EN and EP amperage values to
be set independently. Adjust the ratio of EN to EP amperage to precisely
control heat input to the work and the electrode. EN amperage controls the
level of penetration, while EP amperage dramatically effects the arc cleaning
action along with the AC Balance control.
For example, when welding a thick piece of aluminum, the operator can pour
350 amps of EN into the weld and only 175 amps of EP into the tungsten.
This allows faster travel speeds, faster filler metal deposition, deeper
penetration, and the potential to eliminate preheating. Case studies about
GTAW inverters with independent amperage control suggest that companies
can cut production time by as much as two-thirds.
Increasing EN while maintaining or reducing EP also permits the use of a
smaller-diameter tungsten. This takes heat off of the tungsten and more
precisely directs it into the weld. Companies have reported that this has
allowed them to purchase thinner-diameter electrodes, which are less
expensive than the thicker variety.
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